One method for storing information on magnetic tape uses what is a known as "helical scan" technology. Helical scan tape systems cause information to be recorded in stripes that are diagonal relative to the length of a tape. The helical scan systems use a rotating drum head operating on a slowly driven tape, for high capacity. The tape is wrapped around the rotating drum.
Another method for storing information on magnetic tape uses what is known as "linear recording" technology. Linear recording tape systems cause information to be recorded in multiple parallel tracks that extend in the direction of the length of the tape. Linear recording systems use a stationary head operating on tape that is driven past the head at a speed that is typically much faster than the speed used by helical scan tape systems. With linear recording systems, multiple read/write elements can be employed in a head and can simultaneously operate on the tape. This invention relates to linear recording drive systems.
Servo systems employ information or patterns recorded along a track of the tape to accurately position read/write elements relative to data on the tape. The servo information can be used to accurately position heads relative to the length of the tape (e.g., when searching for a desired position along the length of the tape, such as the start of a file) as well as relative to the width of the tape. Thus, servo patterns on a tape have a characteristic that changes across the width of the tape.
Various servo systems are known in the art. For example, U.S. Pat. No. 5,432,652 (incorporated herein by reference) relates to a magnetic tape that has three evenly spaced-apart longitudinally-extending servo track areas. Four equal-sized longitudinally-extending data track areas are disposed between the servo track areas and between longitudinal edges of the tape and one of said longitudinally-extending data track areas. For track following, all servo track areas are simultaneously sensed for producing one head positioning signal.
U.S. Pat. No. 5,008,765 (incorporated herein by reference) relates to a method for reading or writing data on a tape which has a plurality of data tracks and at least a first dedicated servo track. A multiple channel head is used to access the tracks on the tape. The head is moved proximate one of a plurality of predetermined positions. The channels are located so that, in any one predetermined position of the head, one channel accesses the center of a dedicated servo track on the tape and at least two other channels will access the center of distinct data tracks.
U.S. Pat. No. 5,262,908 (incorporated herein by reference) relates to a tracking control device for a magnetic recording/reproducing apparatus arranged in such a manner that a head unit having a plurality of magnetic heads is successively moved in the widthwise direction of a magnetic tape for switching tracking positions so that data recording/reproducing is, by each of the plurality of magnetic heads, performed along a plurality of data tracks formed on the magnetic tape in parallel to a direction in which the magnetic tape moves.
U.S. Pat. No. 5,574,602 (incorporated herein by reference) relates to a magnetic tape drive. A magnetic head simultaneously senses plural track lateral position indicators to generate a like plurality of independently generated sensed position error signals. The sensed position error signals are combined to provide an output position error signal that drives a positioning system to position the magnetic head laterally of the length of the magnetic tape. The output position error signal represents an average of the position errors indicated by the sensed position error signals. The quality of the sensed position error signal is monitored, eliminating poor quality signals from the output position error signal for maintaining a quality servo control.
U.S. Pat. No. 5,450,257 (incorporated herein by reference) relates to a head-track orienting system for use in magnetic recording tape drives which automatically corrects for misalignment between the head assembly and a recorded servo track on the tape. Using a servo control loop, the system calculates head-track alignment error during operation of the tape drive and either pivots the head assembly or adjusts the tape cartridge to compensate for the error. Transverse head-track positioning mechanisms are also included in the system to locate and maintain a centered position of the heads on the servo track.
One type of servo system is a timing based system. Timing based servo systems are known in the art. In such servo systems, servo bands are written which have a particular servo band configuration. This servo band configuration provides both an indication of position (and speed) in the direction of travel of the tape, and an indication of lateral position of the tape relative to the servo element reading the servo band. The tape drives include a timing based demodulation scheme for sensing the servo information on the tape. This information includes lateral position, tape speed, and encoded data bits. The position of the head relative to the tape width is derived from the relative timing of opposite azimuthally sloped transitions. Readback pulses from the servo code are processed in bursts. A burst is a set of transitions grouped together to generate a predetermined number of pulses when read. The time difference between adjacent bursts represents lateral position and the time difference between alternate bursts represents tape speed. See European Patent Application EP 0690442 A2 for detailed information regarding time based servo systems.
In Adaptive Tape Speed (ATS) systems, tape speed is adapted to the host data rate. This allows fast hosts to transfer data at a high rate while slow hosts will not be forcing the tape drive to stop the tape and reposition back as the data to be written to the tape runs out of the data buffer.
To accomplish ATS, a clock which tracks the tape speed is needed for generating the write clock and to allow the bandwidth of the analog readback filters to track the tape speed. The write clock needs to be a low jitter clock that smoothly tracks the tape speed to ensure that the data bits written to the tape are placed at a uniform spatial distance for any allowed tape speed.
Tape drives include analog readback filters. Readback filters remove noise and are typically Bessel filters or linear phase filters. These filters often use a phase locked loop locked to a clock to set the filter bandwidth. The bandwidth of the readback filter of an ATS tape drive needs to scale with tape speed to create a constant spatial bandwidth because the readback signal bandwidth scales with tape speed and is actually a constant spatial spectrum.
Such a tracking clock could be generated with a Phase Locked Loop (PLL) locked to the servo code written on the tape. The current speed range required by the ATS system is within the range of conventional analog voltage controlled oscillators used in phase locked loops. Using such a phase locked loop would allow filtering or averaging of the speed information from the servo code to offer some immunity to defects and dropouts in the servo signal.
Phase locked loops and frequency locked loops are known in the art and are similar to one another, except that a phase locked loop tracks phase as well as frequency. A phase locked loop includes a phase detector having a first input receiving the incoming message, having a second input, and having an output; a loop filter having an input coupled to the output of the phase detector and having an output; a voltage controlled oscillator (VCO) having an input coupled to the output of the loop filter, and having an output defining an output of the phase locked loop; and a divider having an input coupled to the output of the voltage controlled oscillator and having an output connected to the second input of the phase detector. The phase detector produces an output voltage proportional to the phase difference of two input signals. The loop filter is used to control the dynamics of the phase locked loop. The voltage controlled oscillator produces an AC output having a frequency proportional to input control voltage. The divider produces an output signal having a frequency that is an integer division of the input signal. The loop filter includes a capacitor on a control node of the voltage controlled oscillator.
The term "phase locked loop" as used herein are meant to describe physical structure, not a state of operation. The term "locked" in the phrase "phase locked loop" does not imply that the circuitry is operating, or functioning in a locked condition. Thus, as used herein, "locked" is a term for assisting definition of a particular circuit configuration and is not meant to imply a required state of operation for the circuit.
A disadvantage of this phase locked loop approach is in the fact that the frequency generated by the analog voltage controlled oscillator as a function of the control voltage is unpredictable over temperature, power supply voltage and part to part variations. Due to this unpredictability, a loop is locked using a phase/frequency comparator to adjust the voltage controlled oscillator control voltage until the voltage controlled oscillator phase and frequency matches the desired phase and frequency.
This feedback requirement makes holding the frequency of the voltage controlled oscillator constant over long servo dropouts difficult. If the phase/frequency comparator simply stops receiving pulses from the tape servo code, the voltage controlled oscillator will drift in frequency from the last good data. Methods to switch the phase locked loop into a hold mode are also prone to offsets and drift in the held frequency. Also, if an ATS range greater than the current range was required, the analog voltage controlled oscillator in the phase locked loop would need to have multiple ranges. This would require spaces left on the tape to allow the phase locked loop to settle into the new range.